German Published Patent Application No. 10 2006 021 017 describes an optical position-measuring device having a scanning unit. In the scanning unit, a reflector element is arranged in front of a light source in the scanning beam path. The positioning and arrangement of the reflector element provides that the light source is positioned virtually in the plane of detection, which is an important prerequisite in the utilized scanning principle.
The light source used in this position-measuring device is preferably a point light source, which may be configured, for example, as a semiconductor light source in the form of a so-called VCSEL (vertical cavity surface emitting laser). A problem in such light sources is the possibility of radiation being reflected back from the scanning beam path into the light source. Such back-reflections change the mode spectrum of the semiconductor light source.
Furthermore, interference effects by repeatedly reflected beams of rays in the scanning beam path may be produced as a result of such back-reflections. These also entail collapses in the angular spectrum of the light source. In addition, such back-reflections may have the consequence that the correct positioning of the virtual light source in the scanning beam path is no longer ensured. This affects the tolerance behavior of the position-measuring device negatively.
In Japanese Published Patent Application No. 11-243258 and U.S. Pat. No. 7,193,204, this problem in the use of semiconductor light sources in optical position-measuring devices is recognized. Various design approaches have been provided to circumvent the back-reflection problem.
For example, U.S. Pat. No. 7,193,204 provides for the scanning unit to be tilted slightly with respect to the scanned measuring graduation in order to prevent back-reflections or at least to minimize them.
To avoid these disadvantages, Japanese Published Patent Application No. 11-243258 provides for a lambda quarter disk to be arranged in the scanning beam path, which prevents light from being reflected at a polarization-altering orientation back into the semiconductor light source. These design approaches, however, are on the one hand relatively costly, while on the other hand they intervene in the actually ideal scanning beam path such that one must pay the price of a deterioration of the optimal optical conditions in relation to the scanning process. This is true particularly if a tilting between the scanning unit and the measuring graduation is provided, for example.